1. Field of the Invention
The present invention relates to a driving system for a four-wheel drive working vehicle comprising a hydrostatic transmission (an HST) for driving main-driving wheels, a hydraulic pump, and one or more hydraulic motors for driving sub-driving wheels, wherein output power from the hydraulic motor of the HST for the main-driving wheels is partly transmitted to the hydraulic pump for the sub-driving wheels through another transmission. The present invention also relates to a construction for decelerating the main-driving wheels, wherein a working machine is provided in front of a vehicle body.
2. Related Art
Japanese Utility Model Laid-Open No. Sho 60-32127, for example, discloses a conventional four-wheel drive vehicle provided with an HST. An engine drives a hydraulic motor through a hydraulic pump fluidly connected with the hydraulic motor. The hydraulic motor transmits power to a differential gear unit through a mechanical sub-transmission for the purpose of driving main-driving wheels (rear wheels or front wheels). The hydraulic motor also partly transmits power to another differential gear unit through a transmitting shaft for the purpose of driving sub-driving wheels (front wheels or rear wheels).
Also, Japanese Laid-Open No. Hei 9-121645, for example, discloses a conventional working vehicle having a working machine such as a mower provided either in front of the vehicle body or at its venter portion between its front wheels and rear wheels.
In this conventional construction, the rotary speed of the sub-driving wheels is different from that of the main-driving wheels because the main-driving wheels receive the transmitted power through the sub-transmission. Thus, a problem exists whereby the faster wheels, which are either the main-driving wheels or the sub-driving wheels (the front wheels or the rear wheels), drag the other wheels and a working machine provided on the vehicle. This can cause significant injury to the ground surface (e.g., of a farm), especially if the working vehicle is provided with a mower.
Even if the rotary speeds of the main-driving wheels and the sub-driving wheels (front and rear wheels) are equalized, the four-wheel drive working vehicle turns around an inside main-driving wheel. Therefore, the rotary speed of both sub-driving wheels, which are rather apart from the inside main-driving wheel, slows down so as to be dragged. When the working vehicle travels in two-wheel drive, where only the main-driving wheels are driven, if one of either the left or the right main-driving wheels runs idle, the other main-driving wheel slips because it cannot receive the driving power. This also can cause injury to the ground surface.
For the latter construction, a plurality of mowers are occasionally juxtaposed on a lateral row (arranged on in the center, one on the left and one on the right, for example), so that they can cut the grass in a wide lateral range. In this case, the mowers overlap one another when viewed from the front, so as to leave no uncut grass.
Consequently, these mowers do not overlap when viewed from the side. This causes the size of the working vehicle to increase longitudinally. Furthermore, a center mower of the arrangement must be vertically and movably suspended by a long arm. Accordingly, it is necessary to reinforce the strength of the arm and the lifting power thereof, especially when the center mower is disposed at the fore-end of the vehicle body (before left and right mowers). Additionally, for preventing the vehicle with the mowers from turning along a large turning radius, the overlapping portions of the center mower with the left and right mowers are necessarily lengthened, thereby causing much unevenness of the cut between the portion where the mowers overlap and the portion where they do not overlap.
Where the mower is disposed at the center portion of the working vehicle between the front wheels and the rear wheels, a transmitting shaft is necessarily interposed between the output shaft of the hydraulic motor and the differential gear unit of the sub-driving wheels (front wheels or rear wheels), thereby restricting the space for disposal of the mower.
The first object of the present invention is to provide a driving system which can prevent a working vehicle that is provided with a working machine such as a mower from injuring the ground surface (e.g., of a farm), especially while the working vehicle is turning.
To achieve the first object, in the case of a four-wheel drive working vehicle, a first hydraulic pump driven by the power of an engine and a first hydraulic motor for driving the main-driving wheels are fluidly connected with each other through a closed fluid circuit. This arrangement allows the rotary speed of the sub-driving wheels to correspond to the rotary speed of the main-driving wheels and prevents the working vehicle from dragging the working machine, especially during turning. An output shaft of the first hydraulic motor is drivingly connected with a transmission, so that power transmitted through the transmitting mechanism is given to a first differential gear unit for driving the main-driving wheels and a second hydraulic pump for driving both sub-driving wheels. The second hydraulic pump is fluidly connected to a second hydraulic motor, which is drivingly connected with a second differential gear unit for the sub-driving wheels. The second hydraulic pump or the second hydraulic motor has variable displacement. A displacement adjusting means like a swash plate of the hydraulic pump or the second hydraulic motor is connected with a steering operating tool, so that the rotary speed of the sub-driving wheels can correspond to the degree of turning operation of the steering operating tool.
Alternatively, the second hydraulic pump may be fluidly connected with a pair of second hydraulic motors, which respectively drive left and right axles of the sub-driving wheels without the second differential gear unit. The second hydraulic pump or the pair of second hydraulic motors have variable displacement. A displacement adjusting means like a swash plate of the hydraulic pump or those of the pair of second hydraulic motors are connected with a steering operating tool, so that the rotary speed of the sub-driving wheels can correspond to the degree of turning operation of the steering operating tool.
In the construction wherein the pair of second hydraulic motors are provided, the second hydraulic pump and the pair of second hydraulic motors may also have fixed displacement. In this case, the steering operating tool is connected with a flow control valve unit by oil passages interposed between the second hydraulic pump and the pair of second hydraulic motors, such that the oil displacements of the pair of second hydraulic motors can be controlled by turning operation of the steering operating tool.
The second object of the present invention is to enable a two-wheel drive working vehicle employing the above mentioned driving system to quickly recover from being stuck due to the slipping of both or either of the main-driving wheels while the vehicle is turning.
To achieve the second object, when the working vehicle employing the above mentioned driving system, which can be switched between four-wheel drive and two-wheel drive, is set in two-wheel drive, the driving power of the output shaft of the first hydraulic motor, which has been transmitted through the transmission, is given to the main-driving wheels and is also given to the sub-driving wheels through a clutch. The clutch acts as an over running clutch or a manual clutch, and is interposed between an output shaft of the first hydraulic motor and an input shaft of the second hydraulic pump or on the output side of the second hydraulic motor (output sides of the pair of second hydraulic motors).
The third object of the present invention is to enable a working vehicle employing the driving system of the first object to be light and to be longitudinally short or laterally narrow, thereby providing a small turning radius. Additionally, the object is to allow a working vehicle loaded in front of its vehicle body (its pair of front wheels) with a working machine such as a mower to provide the support and the driving power for vertical moving thereof.
To achieve the third object, the driving system for a working vehicle is further constructed so that both of the output shafts of the first hydraulic motor and the transmission, which are contained in a transmission casing, are disposed in a longitudinal direction of the vehicle so as to drive the first differential gear unit for the main-driving wheels through bevel gears. This construction reduces the lateral width of the transmission casing. Alternatively, both of the output shafts of the first hydraulic motor and the transmission are disposed in a lateral direction of the vehicle so as to drive the first differential gear unit for the main-driving wheels, thereby reducing the longitudinal length of the transmission casing.
Alternatively, in cases where the working machine is disposed in front of the vehicle body, a pair of decelerator casings are disposed respectively on both outer lateral sides of the left and the right front axle casings, so as to project substantially forward in the horizontal direction. Wheel shafts are supported respectively by front portions of the decelerator casings and the pair of main-driving wheels are attached respectively to the wheel shafts. The working machine is disposed between the left and the right decelerator casings, thereby reducing the longitudinal length of the vehicle body and providing the support and the driving power for its vertical moving.
The fourth object of the present invention is to provide a working vehicle with an expanded space at the center portion of the vehicle body between its front and rear wheels for disposal of a working machine such as a mower.
To achieve the fourth object, the second hydraulic pump for driving the sub-driving wheels, which receives the output power of the first hydraulic motor through the transmission, is fluidly connected without a transmitting shaft to the second hydraulic motor, which is drivingly connected with the second differential gear unit for the sub-driving wheels. Also, each of the decelerator casings contains a reduction gear train interposed between the first differential gear unit, which is contained in the front axle casing, and each of the main-driving wheels, so as to reduce the ratio of deceleration of the first differential gear unit, thereby compacting the front axle casing. Thus the space at the center portion of the vehicle body between the front and rear wheels can be advantageously expanded for disposal of the working machine.